Moving a vertex doesn't move the vertex, it just makes another
An additional ring of vertices might have already been created by mistake while you were modelling .This usually happens if you had selected the ring of vertices and tried to extrude by pressing 'E' but didn't move those newly created vertices.If a vertex is placed at the same location as another, then they are called 'doubles', as in double vertices.

Inorder to remove them select all the vertices in edit mode, press space bar to get the search bar and type 'remove doubles'. Alternatively, you can press 'w' in edit mode and find the 'remove doubles' option.


Once you click that, on the top menu bar, you will see info about the doubles that have been removed.